The well-characterized spatial distributions of the transcripts from several Drosophila segmentation genes provide molecular markers which can be used to examine the determination of the segment pattern in early embryos. Tailless (tll) is a zygotic lethal mutation, the phenotype of which is observed by 9 hr of embryogenesis and includes the absence of segments A8, A9, and A10 and a decrease in the procephalic lobe (Strecker et al., Dev. Biol. 113, 64-76, 1986). To establish whether this effect of the tll mutation is due, as proposed previously by Strecker et al., to a reprogramming of the blastoderm fate map, we hybridized probes for the segmentation genes fushi tarazu (ftz) and hairy (h) to whole embryos. The transcripts of these genes show an altered distribution in tll embryos as early as nuclear cycle 14, indicating that the tll gene acts on cellular determination at the blastoderm stage, and is required for normal expression of the ftz and h genes. We obtained more precise information about the alterations in the blastoderm fate map by measuring the position of ftz protein stripes in wild-type and tll embryos. From the results reported here and previously, we conclude that the tll mutation results in a deletion of anterior and posterior ectodermal positional values, concomitant with an expansion of the remaining fate map.